Actually, and this has been discussed at length in other threads, you are correct. The TNG ships all seem to be a couple of points off (Intrepid included...not sure where you're getting the 15 points, though). Don says that this was done after the fact (he said wrote them up so that they fit the cost, but they were changed somewhere in the editing process, IIRC, and thusly ended up two points over).
TOS ships all appear to be 5-10 points under cost (and I've actually followed suit with my TOS ship designs), and the TNG+ ships are all a couple of points over...I think the only exception is the D'Deridex, but that's because we really don't know the cost of the plasma weapon.
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz